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Theoretical Foundations of Multimedia Chapter 3 Monitors n Refresh rate — the frequency at which the phosphors are excited n Normally the refresh rate is givenin Hertz For flicker-free images 75 Hz or faster is desirable For flicker-free images 75 Hz or faster is desirable n The refresh rate for a projector needs to be coordinated with the monitor

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Theoretical Foundations of Multimedia Chapter 3 Monitors n The digitized image to be displayed must be stored in a buffer n The stored image is said to be “bit-mapped,” because, for monochrome images, the map used just one bit per pixel n Multimedia monitors use 24 bits per pixel (8 for each color); can define >16 million colors

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Theoretical Foundations of Multimedia Chapter 3 A Good Multimedia Monitor n Large enough for comfortable viewing, probably 15” or greater n Pixel size of no more than 0.28mm n Refresh rate of at least 75 Hz n Capable of displaying 24-bit color n Designed for the CPU and operating system n Ergonomically comfortable and attractive

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Theoretical Foundations of Multimedia Chapter 3 Alphanumeric Keyboards n For entering commands, text, and data n Each key is a switch that closes when it is depressed, sending a code to the CPU n The arrangement of the keys may vary n The most common is QWERTY n Another arrangement is Dvorak

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Theoretical Foundations of Multimedia Chapter 3 Choosing a Keyboard n Does it include all of the needed characters, including command keys? n Is it ergonomically comfortable and safe, preventing repetitive stress syndrome?

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Theoretical Foundations of Multimedia Chapter 3 E e E E e e E E e E e e E E E e e E e e It is relatively easy for a human to recognize each of these characters as the letter “e.” For the pattern recognition logic in OCR software, this is very difficult. Optical Character Recognition (OCR)

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Theoretical Foundations of Multimedia Chapter 3 Digital Cameras and Scanners n Real Image — n Real Image — a portion of what is physically present in nature Digital Image — n Digital Image — a representation of a real image in which individual points are encoded to represent the wavelength and intensity of light at that point Still Image — n Still Image — a single snapshot of an instant; may be real or digital Motion Image — n Motion Image — a sequence of images that, when viewed consecutively at the appro- priate rate, gives the impression of con- tinuous motion; may be digital or analog

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Theoretical Foundations of Multimedia Chapter 3 Digital Cameras and Scanners n Quality of the optics and the scanning mechanism, which determines focus Precision of the photosensitive cells, which determines the accuracy of the encodingof intensity and wavelength data n Precision of the photosensitive cells, which determines the accuracy of the encodingof intensity and wavelength data Resolution of the instrument in dots per inch, which determines graininess n Resolution of the instrument in dots per inch, which determines graininess Amount of storage available, which deter- mines the total size of an image that canbe digitized n Amount of storage available, which deter- mines the total size of an image that canbe digitized Image quality depends on the:

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Theoretical Foundations of Multimedia Chapter 3 Video Cameras and Frame Grabbers n Video cameras are similar to digital cameras n Except that a video camera takes image after image continuously n The output from many video cameras is analog and requires digitizing circuitryto make the image usable in a computer n Digital camcorders are now available n Frame grabber software allows the capture ofa single still image from the video stream n Frame grabbed images are of rather low resolution, however, <80-90 dots/inch

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Theoretical Foundations of Multimedia Chapter 3 Microphones and MIDI Keyboards n For input of sound n Microphones capture sound waves from theair as an analog signal n The analog signal must be digitized to be stored and then replayed by the computer n Digitizing at <10,000 Hz is adequate for speech; 20,000 Hz is needed for music n MIDI keyboards usually look like piano key- boards with extra switches and controls n MIDI keyboards encode and transmit musical information according to the MIDI standard

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Theoretical Foundations of Multimedia Chapter 3 Inputting Positional Information n Specifying a point on a surface requires two dimensions, as with latitude and longitude n A third dimension could be added, as with altitude n For multimedia, what is commonly needed is position on the monitor in terms of left-right (X) and up-down (Y) distances

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Theoretical Foundations of Multimedia Chapter 3 Inputting Positional Information n X and Y coordinates are obtained relative to a fixed point, usually one corner of the screen n The coordinates are entered in analog form as output from roll- ing wheels inside a device such as a mouse n The analog values are digitized to specify the X and Y coordinates

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Theoretical Foundations of Multimedia Chapter 3 The Mechanism of a Mouse

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Theoretical Foundations of Multimedia Chapter 3 CD-ROMs CD-ROMs n Digital format n Write once, read many times n A rewritable version (CD-RW) is available, but not in common use n Information is “written” by burning tiny holes in the disk surface with a laser n The hole pattern is read by a laser and inter- preted as the bits comprising the data n Can store megabytes of data; about 300,000 pages of double-spaced text, or more than an hour of high fidelity sound

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Theoretical Foundations of Multimedia Chapter 3 Creating Multimedia CD- ROMs n Requires a hard disk large enough to store ~650 megabytes of data to be writtento the CD n Requires a CD-ROM recorder that writes the data to the blank CD using a laser n The developer creates the multimedia mater-ial, stores it on the hard disk, and then tests it as completely as possible n When the material is in final form, it is written to the blank CD as if it were being copied from one disk to another

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Theoretical Foundations of Multimedia Chapter 3 Video Disks or Laser Disks n Much larger than a CD-ROM; ~12” in diameter n Hold ~54,000 video frames per side n Hold ~30 minutes of video per side n Read-only n Analog format n Requires a conversion board to be used with a computer n Excellent for large-scale, video-based multimedia projects

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Theoretical Foundations of Multimedia Chapter 3 VR Head-Mounted Display n Parallax — the apparent change in position of a stationary object when viewed from slightly different positions n A person’s eyes each see a slightly different view of an object n As the brain receives these two images, it interprets the the distance to the object in terms of the difference in position of the object in the two images n Parallax can be used to fool the brain into “seeing” images as being at various distances

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Theoretical Foundations of Multimedia Chapter 3 Demonstrating Parallax Pencil Looking at a pencil aligned What is seen using with the corner of a room both eyes What is seen with right eye covered left eye covered

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Theoretical Foundations of Multimedia Chapter 3 Stereoscope Courtesy of Special Collections, M. I. King Library, University of Kentucky

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Theoretical Foundations of Multimedia Chapter 3 Parallax Problems with VR Head-Mounted Displays n Images may not be perfectly realistic, especially with motion images n When the observer’s head moves and the eyes are refocused, muscle receptor feedback data does not correlate with visual cues n The perspective is always that of the camera, never the viewer’s eyes n A viewer motion feedback mechanism is needed to change the perspective n This all contributes to “cybersickness ”

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Theoretical Foundations of Multimedia Chapter 3 VR Aural Output n Refer to the discussion in chapter 2 regarding the perception of sound n Two key factors u Localization u Identification n The brain interprets differences in the signals it receives from the two ears in a manner analogous to binocular vision n For multimedia sound to be completely realistic, it requires head-position sensing feedback and enormous computational power — not practical for most multimedia

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Theoretical Foundations of Multimedia Chapter 3 VR Position Sensing n A point in space is defined in terms of distance along three mutually perpen-dicular axes, usually termed X, Y, and Z n Motion is defined in terms of changes in position, which requires six parameters n Devices that can sense and record motion are termed six-degrees-of-freedom (6-DOF) devices

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Theoretical Foundations of Multimedia Chapter 3 VR Position Sensing n Sensor output from a 6-DOF device can be u continuous u polled, or sent only upon request n Parameters to consider in evaluating a tracking device: u Lag or Latency — the delay between the actual time of the motion and when it is available as input data; should be <50 mSec u Update rate — Rate at which measurements are made; should be as fast as possible u Precision or accuracy of the measurements u Range over which the sensors operate u Rejection of interference

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Theoretical Foundations of Multimedia Chapter 3 Natural Language Processing n Only the son praised his sister. n Only the son praised his sister. (The rest of the family did not.) n The only son praised his sister. n The only son praised his sister. (There was just one son.) n The son only praised his sister. n The son only praised his sister. (He never found fault with her.) n The son praised only his sister. n The son praised only his sister. (But never anyone else.) n The son praised his only sister. n The son praised his only sister. (He had just one sister.) n The son praised his sister only. n The son praised his sister only. (In this instance, he praised no one but her.)

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Theoretical Foundations of Multimedia Chapter 3 ASCII and Unicode n ASCII is limited because it is only a 7- or 8-bit code; even using “escape sequences” only a small number of characters can be encoded n Unicode is a 16-bit code that can encode many primary scripts plus special character sets known as secondary scripts

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Theoretical Foundations of Multimedia Chapter 3 Modems and Network Interfaces n Start bits u Opposite bit from the system idle state u Necessary to alert the receiver to the beginning of a new character Stop bits n Stop bits u Provide a short delay at the end of each character to give the receiver enough time to convert from serial to parallel n Error-checking codes u Parity bits, CRC bits, etc. u Discussed in chapter 6

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Theoretical Foundations of Multimedia Chapter 3 Modems and Network Interfaces n Transmission rate u Internal transfer rates are much faster than data rates over networks u The interface needs to “interrupt” the computer when it has new data, not keepit from doing other processing whiledata is being received n Transmission form u Connection via a telephone line requires a modem (MOdulator-DEModulator) to translate the internal data transfer format into an audio signal, and vice- versa

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Theoretical Foundations of Multimedia Chapter 3 Modems and Network Interfaces Use of modems and telephone lines for connectivity